Close students of intelligence history will want to read both volumes, which neatly represent the respective virtues of authorized and unauthorized history. As the authorized historian, Jeffery enjoyed privileged access to classified Service archives that no other writer is likely to obtain for years to come. But he was also subject to official restrictions on what he was permitted to publish. So, for example, he could not identify any agents who had not already been publicly identified nor could he tell their stories if doing so would result in their identification.

“Six,” the unauthorized history by veteran intelligence reporter Michael Smith, ranges more widely (though it ends a decade earlier in 1939), taps into foreign archives and private, non-governmental collections, and is subject to no such prior restrictions on disclosure.

The tales of the Service’s early years, now nearly a century old, are vividly told by author Smith, whose book is full of striking observations and asides. Trainspotting in World War I and the early confrontation with Soviet intelligence, among other topics, are treated in this volume, which ends at the dawn of World War II. “Six” has not yet been published in the U.S. but is available from Amazon.com in the UK.